Nucleophilicity refers to the ability of a species to donate an electron pair to form a new bond. In the case of KI (potassium iodide) and KCl (potassium chloride), KI is considered a better nucleophile for a couple of key reasons:
First, the nucleophile’s ability to attack a positive center in a reaction greatly depends on its size and the ability to polarize. Iodide ion (I–), from KI, is larger and more polarizable than chloride ion (Cl–), from KCl. This means that I– can better stabilize the negative charge when it approaches a positive center, making it more reactive as a nucleophile.
Second, the bond strength between the potassium ion and iodide is weaker compared to that of potassium and chloride. This weaker bond makes the iodide ion more willing to dissociate and participate in nucleophilic reactions. In contrast, the stronger bond in KCl makes Cl– less available for nucleophilic attack.
In summary, the larger size and greater polarizability of the iodide ion, along with the weaker bond in KI compared to KCl, contribute to KI being a better nucleophile. This characteristic is especially relevant in organic and inorganic chemistry where nucleophiles are vital in various reactions.